Polyolefin microporous membranes are used in various applications such as battery separators, separation membranes for electrolytic capacitors, various filters, moisture-permeable waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. When a polyolefin microporous membrane is used as a battery separator, particularly as a lithium ion battery separator, the performance of the membrane is highly responsible for battery properties, battery productivity, and battery safety. Therefore, the membrane requires excellent permeability, mechanical properties, heat shrinkage resistance, shutdown property, meltdown property, and the like. For example, if the membrane has low mechanical strength, when it is used as a battery separator, the voltage of the battery can be reduced by a short circuit between electrodes.
To improve the physical properties of polyolefin microporous membranes, studies have been made to improve material composition, stretching conditions, heat treatment conditions, and the like.
For example, a polyolefin microporous membrane having excellent permeation performance and mechanical strength as well as improved moldability, and improved permeability and retentivity of an electrolyte solution has been proposed which is obtained by adding a specific amount of polypropylene to ultra-high molecular weight polyethylene or a composition thereof to cause micro-roughness on the surface of the polyolefin microporous membrane in JP 11-269290 A. The microporous membrane described is a polyolefin microporous membrane comprising a polyolefin composition containing 70 to 95% by weight of polyethylene having a weight average molecular weight of 5×105 or more or a polyethylene composition thereof and 5 to 30% by weight of polypropylene having a weight average molecular weight of 1×104 or more, wherein thickness change between squares with a side length of 1 mm or less adjacent to each other in the surface direction of the membrane surface is ±1 μm or more.
When a membrane is formed by adding a specific polyolefin such as polypropylene to polyethylene and blending them, the polyolefin can segregate on the surface to reduce the near-surface polyethylene content, and as a microporous membrane having such a surface which is able to prevent gas generation during high-temperature storage and decrease in discharge capacity, JP 2004-152614 A has proposed a single-layer microporous membrane containing 50% by weight or more of polyethylene, wherein the near-surface polyethylene content at at least one surface of the membrane is smaller than the average of the whole membrane. That microporous membrane contains polypropylene having a viscosity average molecular weight of 200,000 or more and low molecular weight polypropylene having a viscosity average molecular weight of 50,000 or less each in an amount of 5 to 20% by weight of all the material components.
JP 05-234578 A has proposed a battery separator having excellent mechanical properties and enhanced safety and including an organic electrolyte solution, the separator comprising polyethylene having a specific molecular weight distribution and polypropylene having a weight average molecular weight in a specific range as a polymer component, and a mixture of inorganic fine powder and organic liquid as a membrane forming material, whereby the pressure does not increase during forming a membrane even when the percentage of ultra-high molecular weight portions in the molecular weight distribution of polyethylene is increased. That separator is composed of a microporous membrane comprising a matrix containing polyethylene comprising portions where the molecular weight is 1,000,000 or more in an amount of 10% by weight or more and portions where the molecular weight is 100,000 or less in an amount of 5% by weight or more and polypropylene having a weight average molecular weight of 10,000 to 1,000,000. The amount of the polypropylene is 5 to 45% by weight of the total weight of polyethylene and polypropylene, and the microporous membrane has a thickness of 10 to 500 μm, a porosity of 40 to 85%, a maximum pore size of 0.05 to 5 μm, and a difference between a meltdown temperature and a shutdown temperature of 28 to 40° C.
JP 2011-111484 A has proposed a polyolefin microporous membrane having a bubble point of 400 to 600 kPa and comprising 5 to 50% by mass of a polypropylene component and 50 to 95% by mass of a polyethylene component, wherein the polyethylene component comprises ultra-high molecular weight polyethylene, and the temperature difference between the melting point of the polyethylene component (Tme) and the melting point of the polypropylene component (Tmp), Tmp−Tme, is more than −20° C., but less than 23° C. JP '484 states that a polyolefin microporous membrane suitable as a separator that can simultaneously achieve oxidation resistance and cycle characteristics can be provided.
WO 2007/015416 has proposed a polyolefin microporous membrane comprising polyethylene and polypropylene having a viscosity average molecular weight of 100,000 or more, wherein the polypropylene is contained in an amount of 4 wt % or more, and the number of terminal vinyl groups per 10,000 carbon atoms in the polyolefin constituting the microporous membrane as determined by infrared spectroscopy is at least two. That microporous membrane has achieved both membrane rupture resistance and a low heat-shrinking property, and has excellent fuse properties and a uniform thickness.
In addition to the publications described above, JP 2001-183432 A, JP 2002-105235 A, and WO 2005/113657 have also proposed a polyethylene microporous membrane into which polypropylene is introduced.
As described in JP '290, JP '614, JP '578, JP '484 and WO '416, attempts have been made to introduce polypropylene into a polyethylene microporous membrane to thereby improve the physical properties of the microporous membrane. However, there has been a disadvantage in that a significant amount of polypropylene needs to be introduced to improve heat resistance or oxidation resistance, and as the amount of polypropylene increases, the permeability-strength balance of the polyethylene microporous membrane is impaired, specifically, the strength is reduced.
The polyethylene microporous membranes into which polypropylene is introduced described in JP '432, JP '235 and WO '657 all had an insufficient permeability-strength balance.
Thus, to ensure the productivity and safety of a battery, it is required to improve oxidation resistance and, at the same time, maintain the excellent permeability-strength balance that a polyethylene microporous membrane has.
Thus, it could be helpful to provide a polyolefin microporous membrane excellent in oxidation resistance, mechanical properties, permeability, and injection of electrolyte.